The present invention relates to aerial devices such as aerial platforms. In particular, the present invention is an aerial device which provides improved safety in raising, lowering, and holding a boom position.
Aerial platforms have found wide use, particularly by public utilities because they enable a workman to have access to overhead power lines, street fixtures, pole-mounted transformers, and other locations remote from the ground. Many of these devices are mounted on the bed of a truck so that they are easily moved from one location to another. The devices typically include a boom, a workman's platform or basket, and a hydraulic control system for raising and lowering the boom and pivoting the platform to maintain the platform level. In some cases, the device includes only a single boom, while other devices have articulated booms with several sections.
Most aerial devices use a hydraulic actuator to raise or lower a boom. It is extremely important that the hydraulic actuator maintain its hydraulic pressure and not collapse while the boom is in an elevated position. This clearly would present a dangerous situation for a workman in the platform or basket.
When a hydraulic actuator, which typically includes a cylinder and a hydraulic piston, is in compression, there is less danger of a loss in hydraulic pressure than when the actuator is in tension. This is because the actuator, when in tension, is relying upon the end seal through which the piston rod extends out of the cylinder to hold pressure. The failure of this seal, which is termed "end gland failure" will cause collapse of the cylinder, and therefore the boom, if the cylinder is holding the boom in position while in tension rather than in compression. The likelihood of leakage and loss of pressure is greatly reduced when the actuator is in compression because there is no similar seal at the opposite end of the cylinder. The only possible leakage is past the piston itself or through the connection of the hydraulic lines to the cylinder.